This is a repository copy of Gradual caldera collapse at Bárdarbunga volcano, Iceland, regulated by lateral magma outflow. White Rose Research Online URL for this paper: http://eprints.whiterose.ac.uk/103058/ Version: Accepted Version Article: Gudmundsson, MT, Jónsdóttir, K, Hooper, A orcid.org/0000-0003-4244-6652 et al. (45 more authors) (2016) Gradual caldera collapse at Bárdarbunga volcano, Iceland, regulated by lateral magma outflow. Science, 353 (6296). aaf8988. ISSN 0036-8075 https://doi.org/10.1126/science.aaf8988 Reuse Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of the full text version. This is indicated by the licence information on the White Rose Research Online record for the item. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing [email protected] including the URL of the record and the reason for the withdrawal request. [email protected] https://eprints.whiterose.ac.uk/ 1 Gradual caldera collapse at Bárdarbunga volcano, Iceland, 2 regulated by lateral magma outflow 3 Magnús T. Gudmundsson1, Kristín Jónsdóttir2, Andrew Hooper3, Eoghan P. Holohan4,5, Saemundur 4 A. Halldórsson1, Benedikt G. Ófeigsson2, Simone Cesca4, Kristín S. Vogfjörd2, Freysteinn 5 Sigmundsson1, Thórdís Högnadóttir1, Páll Einarsson1, Olgeir Sigmarsson1,6, Alexander H. Jarosch1, 6 Kristján Jónasson7, Eyjólfur Magnússon1, Sigrún Hreinsdóttir8, Marco Bagnardi3, Michelle M. 7 Parks1, Vala Hjörleifsdóttir9, Finnur Pálsson1, Thomas R. Walter4, Martin P.J. Schöpfer10, Sebastian 8 Heimann4, Hannah I. Reynolds1, Stéphanie Dumont1, Eniko Bali1, Gudmundur H. Gudfinnsson1, 9 Torsten Dahm4, Matthew Roberts2, Martin Hensch2, Joaquín, M.C. Belart1, Karsten Spaans3, 10 Sigurdur Jakobsson1, Gunnar B. Gudmundsson2, Hildur M. Fridriksdóttir1,2, Vincent Drouin1, 11 Tobias Dürig1, Gudfinna Adalgeirsdóttir1, Morten S. Riishuus1, Gro B.M. Pedersen1, Tayo van 12 Boeckel1 , Björn Oddsson11, Melissa A. Pfeffer2, Sara Barsotti2, Baldur Bergsson2, Amy Donovan12, 13 Mike R. Burton13, Alessandro Aiuppa14 14 15 Revised manuscript for Science – 14 June 2016 16 17 1: Nordvulk, Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík 18 2: Icelandic Meteorological Office, IS-150 Reykjavík, Iceland 19 3: Centre for the Observation and Modelling of Earthquakes, Volcanoes and Tectonics (COMET), 20 School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK 21 4: GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany 22 5: UCD School of Earth Sciences, University College Dublin, Ireland 1 23 24 6: Laboratoire Magmas et Volcans, CNRS-Université Blaise Pascal-IRD, 63038 Clermont- 25 Ferrand, France 26 7: Faculty of Industrial and Mechanical Engineering and Computer Science, University of 27 Iceland, Hjarðarhagi 2-6, 107 Reykjavík, Iceland 28 8: GNS Science, PO Box 30368, Lower Hutt 5040, New Zealand 29 9: Departamento de Sismología, Instituto de Geofísica, Universidad Nacional Autónoma de 30 Mexico, 04510 Ciudad de México, Mexico 31 10: Department for Geodynamics and Sedimentology, University of Vienna, Althanstrasse 14, A- 32 1090 Vienna, Austria. 33 11: National Commissioner of the Icelandic Police, Department of Civil Protection and Emergency 34 Management, Skúlagata 21, 101 Reykjavík, Iceland 35 12: King’s College London, King's Building, Strand Campus, London WC2R 2LS, England, United 36 Kingdom 37 13: University of Manchester, Williamson Building, Oxford Road, Manchester, M13 9PL, UK 38 14: University of Palermo – Piazza Marina, 61 90133, Palermo, Italy 39 2 40 Abstract 41 Large volcanic eruptions on Earth commonly occur with collapse of the roof of a crustal magma 42 reservoir, forming a caldera. Only a few such collapses occur per century and lack of detailed 43 observations has obscured insight on mechanical interplay between collapse and eruption. We use 44 multi-parameter geophysical and geochemical data to show that the 110 km2 and 65 m deep 45 collapse of Bárdarbunga caldera in 2014-15 initiated through withdrawal of magma, and lateral 46 migration through a 47 km long dyke, from a 12 km deep reservoir. Interaction between the 47 pressure exerted by the subsiding reservoir roof and the physical properties of the subsurface flow 48 path explain the gradual, near exponential decline of both collapse rate and the intensity of the 181- 49 day long eruption. 50 ________________ 51 52 Calderas are 1 - 100 km diameter depressions found in volcanic regions of Earth and other planets. 53 They mainly form by collapse of overburden into a subterranean magma reservoir during large 54 volcanic eruptions, including the largest known super-eruptions (1-8). From 1900 AD to 2014, only 55 six cases have been documented and with varying degrees of detail. The collapses of Katmai in 56 1912 and Pinatubo in 1991 occurred during explosive silicic (andesite-rhyolite) eruptions, the 57 largest of the 20th century. The collapses of Fernandina in 1968, Tolbachik in 1975-76, Miyakejima 58 in 2000 and Piton de la Fournaise in 2007 were associated with mainly effusive mafic (basalt – 59 basaltic andesite) intrusive activity and eruptions (2, 9-12). 60 The consensus from field and modelling studies is that caldera collapse progresses from initial 61 surface downsag to fault-controlled subsidence (1, 8, 13, 14). The limited number of modern 62 examples and the scarcity of geophysical data leaves open the question of whether collapse occurs 63 suddenly or gradually during the course of an eruption. The issue of whether collapse drives magma 64 movement and eruption or eruption drives collapse also remains unresolved. Previous geological, 3 65 geophysical, and modeling studies have produced a diverse and inconsistent set of answers to such 66 questions (2, 4, 15, 16). The caldera collapse at Bárdarbunga in central Iceland from August 2014 to 67 February 2015 offers a unique opportunity to address them directly. 68 69 Figure 1. Bárdarbunga and geometry of collapse. A) Map showing the total caldera subsidence 70 (in meters) at the end of collapse in February 2015þ Minor sustained geothermal activity, monitored 71 from aircraft, increased during the collapse with pre-existing ice cauldrons deepening by up to 50 m 72 and new ones forming at the southern margin and to the southeast of the caldera (24). (B) Radio- 73 echo sounding profile from 3 February, 2015, and a cross-section of the caldera with the collapse. 4 74 The pre-collapse topography is obtained by subtracting the subsidence observed at the surface. (C) 75 Modelled changes in ice thickness at the end of February 2015 resulting from ice flow in response 76 to caldera collapse (24). D) NNW-SSE and E) WSW-ESE cross-sections as measured in June 77 2015, corrected for winter snow accumulation in 2014-15, measured in June 2015, and modeled 78 vertical ice flow. Subsidence extends 2-3 km beyond the pre-existing caldera rims (dotted lines) 79 where it amounts to 3-11 m. 80 The Bárdarbunga volcano and the Holuhraun eruption of 2014-15 81 Bárdarbunga volcano (Fig. 1) and its related fissure swarms form a 150 km long volcanic system on 82 the boundary between the North-American and Eurasian tectonic plates. The volcano resides 83 beneath the Vatnajökull ice cap and has a broadly elliptic 13 by 8 km wide and 500-700 m deep 84 caldera with a long axis trending ENE. About 700-800 m of ice fills the caldera (17, 18). Over 20 85 eruptions have occurred on the fissure swarms outside the caldera in the last 12 centuries, including 86 three that produced 1-4 km3 of magma, but no eruptions are known within the caldera in this period. 87 (19). 88 At 4 UTC on 16 August 2014, the onset of intense seismicity beneath the caldera marked the 89 beginning of a major rifting event (20). The seismic activity was mostly located in the SE-corner of 90 the caldera in the first few hours, but it soon began to propagate out of the caldera towards the SE 91 (Fig. 2). After propagating to about 7 km from the caldera rim, fifteen hours after the onset of 92 seismicity (~19 UTC), the moving earthquake cluster took a 90° turn and started migrating towards 93 the NE. In the two weeks that followed, surface deformation and migration of seismicity indicated 94 that a magmatic dike propagated laterally northeastward for 47 km in the uppermost 6-10 km of the 95 Earth’s crust (20, 21). On 31 August, a major effusive eruption began above the far end of the dike; 96 this lasted six months and produced 1.5±0.2 km3 of lava (~1.4±0.2 km3 of bubble-free magma) 97 (22), making it the largest in Iceland (or Europe) since the 1783-84 Laki eruption. Combined with 5 98 the 0.5±0.1 km3 dyke (20), the total volume of identified intruded and erupted magma was 1.9±0.3 99 km3. 100 The Onset of Collapse 101 After the initial seismic activity in the caldera receded late on 16 August, seismicity was relatively 102 minor there until 20 August. At the same time our GPS time-series from stations close to the 103 caldera, suggest that deflation of the magma reservoir started on 16 August (20). On 20 August, 104 caldera seismicity increased progressively with a series of earthquakes of magnitude M4-M5.8 105 occurring in the following days (Fig. 2). The first two events occurred on the southern caldera rim 106 (M4.7 on 20 August and M5.1 on 21 August). Following these earthquakes, three similar magnitude 107 events occurred on the northern rim on 23 August, followed by four events on the southern rim on 108 24-25 August.